1. Field of the Invention
The present invention relates to a vehicle bumper beam, and more particularly to such a vehicle bumper beam capable of maintaining a required strength thereof while providing an impact absorbing member with a crush stroke that is large enough to absorb an impact energy.
2. Discussion of Prior Art
There is widely used a vehicle bumper beam which has an elongated shape and which is to be attached at its longitudinally opposite end portions to respective side members of a vehicle through respective impact absorbing members (see JP-H09-95189A). As an example of such a bumper beam, a bumper beam 10 is shown in FIG. 3 that is a schematic plan view showing a front portion of a vehicle as seen from an upper side of the vehicle. In front end portions of respective right and left side members 12R and 12L, there are disposed crush boxes 14R and 14L as impact absorbing members, respectively. The bumper beam 10 is fixed at its right and left end portions to the respective crush boxes 14R, 14L. FIG. 4 is a cross sectional view taken along line 4-4 of FIG. 3, i.e., a cross sectional view of a vicinity of the right-side attached portion of the bumper beam 10. The bumper beam 10 has a hat shape in its cross section, and the crush box 14R is introduced inside the hat shape so as to be integrally fixed to the bumper beam 10. The bumper beam 10 is symmetrical so that the left-side attached portion is constituted similarly as the right-side attached portion. This bumper beam 10 functions as a reinforcement (reinforcing member) of a bumper and also as an attached member, and is integrally attached to a bumper body 16 formed of synthetic resin or the like. Each of the crash boxes 14R, 14L is provided by a cylindrical member, and is crushed like an accordion upon application of impact thereto in a direction from the vehicle front to the rear, so as to absorb the impact energy for thereby alleviating the impact applied to structural members of the vehicle such as the side members 12R, 12L.
Meanwhile, it is proposed, in JP-2003-146159A, a vehicle bumper beam is formed of a tube material (pipe) that is subjected to a hydroforming, such that the bumper beam has in its central portion a B-shaped cross-sectional configuration providing a high rigidity (strength), and such that its height (a distance by which the bumper beam protrudes forwardly of the vehicle) is reduced as viewed in a direction toward each of its opposite end portions, for thereby restraining squareness of corner portions of the vehicle. In the vehicle bumper beam, although a large bending strength is required in its central portion distant from the opposite end attached portions at which the bumper beam is attached to the impact absorbing members, a strength required in its opposite end portions close to the attached portions is relatively small. Thus, in each of the opposite end portions of the bumper beam, the height may be reduced.
The above-described vehicle bumper beam described in FIGS. 3 and 4 of JP-H09-95189A can be inexpensively manufactured by using a flat-sheet-shaped metallic material, which is subjected to a press working or the like. However, since the bumper beam has a simple hat-like shape in its cross section, the thickness of the sheet has to be increased in order to enable the bumper beam to have a required bending strength (rigidity), thereby resulting in increase in a weight of the bumper beam. Further, the arrangement in which the impact absorbing member (crush box) is introduced inside the hat shape to be integrally fixed to the bumper beam, leads to reduction in a spacing distance D (see FIG. 4) by which the bumper beam is to be moved until it is brought into contact with the side members or other structural members (an adapter plate 64 in FIG. 4), thereby making it impossible to provide a crush stroke required for the crush box to be completely crushed, and causing a possibility that an expected impact-absorbing performance could not be obtained.
On the other hand, in the vehicle bumper beam disclosed in JP-2003-146159A, an excellent bending strength can be obtained, and a sufficient crush stroke is easily obtained owing to the reduction in the height in the opposite end portions. However, the use of the pipe leads to increase in the manufacturing cost. Although it might be possible for a flat-sheet-shaped metallic material to be bent to have a closed cross section, this complicates the manufacturing process, not necessarily leading to sufficient reduction in the manufacturing cost.
Further, it might be possible for a flat-sheet-shaped metallic material to have a M shape in its cross section as a bumper beam 20 shown in FIG. 5, for example, by roll forming or extrusion forming. In this case, the M shape has a higher strength than the hat shape, thereby making it possible to reduce the weight by reducing the thickness of the sheet or reducing the height. Further, it would be possible to manufacture it at a cost lower than the bumper beam of the closed cross section. However, in the bumper beam formed by the roll forming or extrusion forming, since the cross-sectional configuration is constant, the height is made large also in the attached portions at which the bumper beam is attached to the crush boxes, so that the length L of the crush boxes is reduced due to the large height in the attached portions of the bumper beam. The reduction in the length L of the crush boxes reduces the impact-absorbing performance. It is noted that FIG. 5 is a cross sectional view corresponding to FIG. 4.
The present invention was made in the light of the background art discussed above. It is therefore an object of the invention to provide a light-weighted and inexpensive vehicle bumper beam capable of maintaining a required bending strength thereof while assuring a sufficient crush stroke.